This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network or link therebetween. More particularly, this invention relates to a system wherein a document is scanned at a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
Presently available facsimile transmitters and receivers are capable of different transmission modes. For example, the facsimile transmitters and receivers are capable of operating at different scanning rates depending upon the particular resolution required in a transmission so as to achieve different transmission times or durations. The QWIP 1000 facsimile transceiver manufactured by QWIP Systems Division of Exxon Enterprises is capable of different scanning rates to achieve a 4-minute transmission mode as well as a 6-minute transmission mode. These different scanning rates are achieved in the QWIP 1000 facsimile transceiver which comprises a rotatable drum for achieving the scanning motion by maintaining a constant drum speed in both the 4-minute mode as well as the 6-minute mode while varying the rate at which the reading or writing head moves along a direction parallel with the axis of the drum.
Typically, the operator of a facsimile transmitter such as the QWIP 1000 is able to advise the operator of a facsimile receiver of the particular choice of a transmission mode. In other words, the facsimile transmitter operator can examine the document to be transmitted and determine the resolution required while also considering the cost involved in utilizing the transmission channel for a transmission of six minutes versus four minutes. It is, however, desirable in some instances to provide for automatic signaling of the particular transmission mode so as to permit the receiver to automatically respond to and prepare for the transmission without intervention by the operator.
In order to provide for transmission mode signaling, the model 410 transceiver manufactured by the Xerox Corporation frequency modulates the synchronizing signal which is coincident with the juxtaposition between the photodetector and the clamp of a rotating drum on the transceiver. More particularly, a signal is transmitted from a transmitter having a frequency of 1500 Hz. during the synchronizing pulse for a 6-minute transmission mode and a signal of 1250 Hz. is generated during the synchronizing pulse for a 4-minute transmission mode. Since the Xerox 410 provides for a 15 second synchronizing period at the beginning of a transmission mode, it is possible for a receiver to detect the frequency of the synchronizing pulse and select the appropriate scanning rate in a direction parallel to the axis of the 410 drum before the document transmission begins. Typically, the duration of the synchronizing pulse is constant and the occurrence of the pulse relative to the rotation of the drum clamp past the document photodetector is a function of the tolerance on a pulse generator associated with the rotatable drum.
In addition to providing different scanning rate transmission modes, e.g., 4 or 6-minute transmission modes, it is desirable to provide for the selection of other transmission modes.